1. Field of the Invention
The present invention relates to a sewing machine upon which a sewing operation is carried out to secure a predetermined width of a margin to sew up according to a preset margin data.
2. Description of the Prior Art
As shown in FIG. 1, in case, for example, a pocket fabric piece 201 which is another work fabric is sewn on a work fabric 200 for the breast part of a garment or the like, a margin to sew has been secured so far as follows. That is, first a side edge portion 202 of the pocket fabric piece 201 is sewn so as to leave a required width of margin from the side edge 202, the sewing operation is stopped at a position where a required interval l is formed between the side edge portion 202 and another side edge portion 203 adjacent thereto, and then the sewing operation is kept going along the side edge portion 203. Accordingly, the interval may constitute a margin to sew for the side edge portion 203.
Thus each side edge portion of the pocket fabric piece 201 can be sewn securing a required width of margin therefor by repeating the above-described operation. However, it was difficult to keep the interval l constant for the reason that the first needle-drop position could not be kept constant, the work fabric expanded and contracted, and so forth. Thus the sewn goods cannot be finished uniformly, which may result in a depreciation of commercial value of the sewn goods.
To settle the above-mentioned situation, various types of sewing machines were hitherto proposed. These sewing machines are provided with a sensor for detecting the trailing edge of a work fabric on an up stream side of a needle, and a required width of the margin is secured by correcting terminal stitch length after detection of the trailing edge on the sensor. For example, a sewing machine provided with a correction unit for correcting a predetermined number of stitches is disclosed in U.S. Pat. No. 4,381,719. In this type of sewing machine, whether or not stiches are aligned duly will be decided according to a rotational phase of a main shaft at the time of trailing edge end detection on the sensor, and thus the number of stitches in the terminal stitch length is corrected accordingly thereto. Then, another sewing machine for correcting the length of one or plurality of stitches on an instantaneous needle position at the time of trailing edge detection is disclosed in West Germany Pat. No. 3,228,789. Further, such sewing machines as will secure a required width of margin to sew by applying a correction to the stitch number or length are also disclosed in Japanese Pat. Publication No. 38646/1978, U.S. Pat. No. 4,359,953, U.S. Pat. No. 4,404,919. However, in the prior arts, a margin to sew up is set as the frequency of needle drops unexceptionally from the time of trailing edge detection to the shutdown of a sewing machine. Accordingly, for setting a required width of margin, there yet needed is the operation wherein one stitch length is multiplied by the number of stitches at a terminal stitch length, and the value thus obtained is subtracted from the distance between a needle and a sensor, which is extremely troublesome.